Apart from the price, I prefer Udemy courses because I can check ratings and reviews, so I know what I am paying for. With Udacity, it's a bit of a mystery. It's like signing up for an interesting class in college and realizing the professor is bad later. Also, it really bothers me that the names Udemy and Udacity are so similar!!
honestly wish all these MOOC's would just adopt something similar to Udemy. Its just miles better than everything else out there in terms of its video UI, course reviews, pricing, etc. Udemy doesn't have as much deep computer science topics though found elsewhere though.
Udemy is good for beginners, but suffers in quality for most courses.
Ditto for udacity. Although Peter Norvigs course is outstanding